Improve comment explaining metrics selection

nixos/roles/default.nix

@@ -3,6 +3,7 @@
 imports = [
   ./admins.nix
   ./nix_keys.nix
+  ./prometheusNodeExporter.nix
   (modulesPath + "/virtualisation/xen-domU.nix")
   ../modules/impermanence.nix
   ];

nixos/roles/prometheusNodeExporter.nix

@@ -0,0 +1,40 @@
+{config, ...}:
+{
+  imports = [ ];
+  services.prometheus.exporters.node = {
+    enable = true;
+    port = 9100;
+  # Aligned with https://git.rwth-aachen.de/fsdmath/server/prometheus/-/blob/main/node_exporter/etc/default/prometheus-node-exporter
+  # It was compiled along the following steps:
+  # 1. Does the current Debian release supports the collector?
+  # 2. Is the collector depracated in the latest release?
+  # 3. Could you probably use the collected metrics for monitoring or are they useless because they make no sense in our context
+  #    (e.g. power adapter inside a VM, use fibre port connection)?
+    disabledCollectors = [
+      "arp"
+      "bcache"
+      "btrfs"
+      "dmi"
+      "fibrechannel"
+      "infiniband"
+      "nvme"
+      "powersupplyclass"
+      "rapl"
+      "selinux"
+      "tapestats"
+      "thermal_zone"
+      "udp_queues"
+      "xfs"
+      "zfs"
+    ];
+    enabledCollectors = [
+      "buddyinfo"
+      "ksmd"
+      "logind"
+      "mountstats"
+      "processes"
+    ];
+  };
+  networking.firewall.allowedTCPPorts = [ 9100 ];
+  environment.persistence.${config.impermanence.name}.directories = [ "/var/lib/${config.services.prometheus.stateDir}" ];
+}